Process and apparatus for forming plastic sheet

ABSTRACT

Disclosed is an apparatus for formation of high quality plastic sheet in a continuous fashion. Also disclosed are a variety of optical and electronic display applications for high quality plastic sheet produced in a continuous fashion.

BACKGROUND OF THE INVENTION

The present invention relates to a process and apparatus for formingplastic sheet. In particular, the present invention relates to a processand apparatus for forming plastic sheet having low residual stress andhigh surface quality. Plastic sheet formed according to the process ofthe present invention is particularly useful in optical and electronicdisplay applications, such as, for example, optical windows, opticalfilters, recording media, and liquid crystal displays (“LCD”).

Sheets of optical quality glass or quartz are used in electronic displayapplications as “substrates.” In such applications, a “substrate” is asheet of material used to build an electronic display. Such substratescan be transparent, translucent or opaque, but are typicallytransparent. In general, such sheets have conductive coatings appliedthereto prior to use as substrates. Such substrates often have stringentspecifications for optical clarity, flatness and minimal birefringence,and typically must have high resistance to gas and solvent permeation.Mechanical properties such as flexibility, impact resistance, hardnessand scratch resistance are also important considerations. Glass orquartz sheets have been used in display applications because thesematerials are able to meet the optical and flatness requirements andhave good thermal and chemical resistance and barrier properties;however, these materials do not have some of the desired mechanicalproperties, most notably low density, flexibility and impact resistance.

Because of the mechanical limitations of glass or quartz sheet inoptical or display applications, it is desirable to use plastic sheet insuch applications. Although plastic sheets have greater flexibility, aremore resistant to breakage, and are of lighter weight than glass orquartz sheets of equal thickness, it has been very difficult to produceplastic sheet having the requisite optical specifications needed for usein optical and display applications at reasonable costs. Moreover, manytypes of plastic sheet undergo unacceptable dimensional distortion whensubjected to substrate processing conditions during manufacture of thedisplay devices, particularly with respect to temperature.

There are several commercially utilized methods for producing plasticsheet and film, including casting, extrusion, molding, and stretchingoperations. Of these methods, several are not suitable for producinghigh quality plastic sheet. As used throughout this specification, theterm “high quality” is used to describe plastic sheet having thefollowing characteristics: low surface roughness, low waviness, lowthickness variation, and minimal amount of polymer chain orientation(for example, as measured by asymmetric physical properties,birefringence or thermal shrinkage).

For example, injection molding is likely to produce high amounts ofpolymer chain orientation, especially for thin sheets (i.e., 1 mmthickness or less), due to the flow of molten plastic into the mold,which unacceptably increases birefringence for polymers withnon-negligible photoelasticity (stress optic) coefficients. Injectioncompression molding is an improved molding process which allowssqueezing of the polymer after injection for the purpose of improvingsurface quality and reducing polymer chain orientation. However, evenwith these improvements, injection compression molding has limitedability to produce high quality sheet.

Compression molding and press polishing may be used to produce sheetswith good surface quality; however, the squeezing flow inherent in suchprocesses results in polymer chain orientation which results inunacceptable shrinkage during thermal cycling. Moreover, these processesare not continuously operable and therefore increase labor andproduction costs.

Stretching operations (for example, for the production of uniaxially- orbiaxially-oriented films) and blown film extrusion inherently introducelarge amounts of polymer chain orientation and are unsuited for theproduction of high quality plastic sheet.

Solvent casting can be used to produce high quality film; however, thereare practical limitations to the maximum film thickness which can beproduced by this method. In addition, the solvent used in the castingmust be removed after formation of the sheet.

Sheet extrusion is run as a continuous operation, but this processintroduces unacceptable polymer chain orientation due to the nature ofthe polymer flow in the die and between the polished rollers in the rollstack.

There is therefore a continuing need for a method for producingrelatively inexpensive, high quality plastic sheet in a continuousfashion, wherein the resultant plastic sheet is capable of use as asubstrate in optical and electronic display applications.

STATEMENT OF THE INVENTION

The present invention is directed to an optical storage mediumcomprising one or more layers of high quality plastic sheet; areflective or semireflective layer disposed on at least one side of thesheet; and optionally a protective layer disposed on at least one sideof the sheet; wherein the plastic sheet is produced by the processcomprising the steps of: a) providing molten plastic resin; b) directingthe molten plastic resin to an overflow die having an inlet and anoutlet; c) shaping the molten plastic resin into a molten web using saidoverflow die; d) guiding said molten web away from said overflow die;and e) cooling said molten web to form a solid sheet.

The present invention is also directed to an apparatus for producinghigh quality plastic sheet, comprising: a) a source for providing moltenplastic resin; b) an overflow die having a length and a width,comprising: a substantially egg-shaped cross-section culminating in anapex, a conduit opening, and a metering arrangement connected with saidconduit opening, wherein the molten plastic resin flows into the diethrough the conduit opening, out of the die through the meteringarrangement, and around the sides of the die to form a molten web atsaid apex; c) means for delivering said molten plastic resin from saidsource to said overflow die; d) guidance means for guiding said moltenweb away from said overflow die; e) means for filtering disposed betweensaid delivery means and said overflow die; and f) means for mixingdisposed between said filter means and said overflow die.

The present invention is further directed to a substrate for liquidcrystal display comprising a high quality plastic sheet having anelectronic component disposed on at least one side of the sheet; whereinthe plastic sheet is produced by the process comprising the steps of: a)providing molten plastic resin; b) directing the molten plastic resin toan overflow die having an inlet and an outlet; c) shaping the moltenplastic resin into a molten web using said overflow die; d) guiding saidmolten web away from said overflow die; and e) cooling said molten webto form a solid sheet; provided that when the plastic resin is apolycarbonate, it does not contain as bisphenol components (1)1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; (2) a mixture of2,2-bis(4-hydroxyphenyl)propane and1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane; a blend of (1) and(2); or a blend of (1) or (2) with a second polycarbonate containing asa bisphenol component 2,2-bis(4-hydroxyphenyl)propane.

The present invention is further directed to a magnetic storage mediumcomprising a high quality plastic sheet having a magnetic layer disposedon at least one side of the sheet; wherein the plastic sheet is producedby the process comprising the steps of: a) providing molten plasticresin; b) directing the molten plastic resin to an overflow die havingan inlet and an outlet; c) shaping the molten plastic resin into amolten web using said overflow die; d) guiding said molten web away fromsaid overflow die; and e) cooling said molten web to form a solid sheet.

The present invention is further directed to a 3-dimensional opticalstorage medium comprising a high quality plastic sheet having dispersedtherein one or more pigments, dyes or mixtures thereof, wherein thepigments, dyes or mixtures thereof undergo a change in opticalproperties locally upon exposure to light; wherein the optical storagemedium is produced by the steps of: a) providing molten plastic resin;b) directing the molten plastic resin to an overflow die having an inletand an outlet; c) shaping the molten plastic resin into a molten webusing said overflow die; d) combining the dye, pigment or mixturethereof with the molten plastic resin prior to shaping the molten resininto a web using said overflow die; e) guiding said molten web away fromsaid overflow die; and f) cooling said molten web to form a solid.

The present invention is still further directed to a method of preparingan optical storage medium having information encoded thereon comprisingthe steps of producing a high quality plastic sheet by the processcomprising the steps of: a) providing molten plastic resin; b) directingthe molten plastic resin to an overflow die having an inlet and anoutlet; c) shaping the molten plastic resin into a molten web using saidoverflow die; d) guiding said molten web away from said overflow die; e)cooling said molten web to form a solid sheet; coating the sheet with apolymer film; and encoding information on the coated sheet by embossing.

The present invention is further directed to a light management filmcomprising a high quality plastic sheet having a structured surface,wherein the film is produced by the steps of: a) providing moltenplastic resin; b) directing the molten plastic resin to an overflow diehaving an inlet and an outlet; c) shaping the molten plastic resin intoa molten web using said overflow die; d) guiding said molten web awayfrom said overflow die; e) cooling said molten web to form a solidsheet; and f) providing a structured surface to the sheet before, duringor after cooling.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a frontal view of a typical apparatus of the presentinvention.

FIG. 2 is a side view of the apparatus of FIG. 1.

FIGS. 3A-3C are close-ups of overflow die 20.

FIG. 3A is a perspective view of the die with heating manifold attached.

FIG. 3B is a top view of the die; and

FIG. 3C is a side view of the die.

FIG. 4 is a cross-sectional view of overflow die 20.

FIGS. 5-7 are alternate embodiments of the overflow die of the presentinvention.

FIG. 5 illustrates an overflow die having a series of holes in place ofthe slot 22 of die 20;

FIG. 6 illustrates an overflow die having a non-tapering slot; and

FIG. 7 illustrates an overflow die having a “coat hanger” arrangement.

DETAILED DESCRIPTION OF THE INVENTION

As used in this specification, the following terms have the followingdefinitions, unless the context clearly indicates otherwise. “Glasstransition temperature” or “Tg” is the midpoint of the narrowtemperature range over which polymers change from being relatively hardand brittle to relatively soft and viscous (rubbery). “Plastic” refersto polymer, such as thermoplastic polymers, which can form sheets. Theterms “polymer” and “resin” are used interchangeably throughout thespecification. “Sheet” refers to a sheet having a thickness of about 25mm or less, and is intended to include “films” (sheets having thicknessof <0.5 mm). “Shrinkage” refers to an irreversible dimensional changethat occurs in a sheet subjected to a heat-cool cycle. The terms“Bisphenol A” and “2,2-bis(4-hydroxyphenyl)propane” are usedinterchangeably throughout the specification. “Bisphenol Apolycarbonate” refers to a polycarbonate containing bisphenol A andphosgene. The following abbreviations are used in the specification:cm=centimeter(s); mm=millimeter(s); NE=nanometer(s); μ=micron(s)(micrometers); g=gram(s); mL=milliliters; Pa=Pascals; kPa=kiloPascals;Pa-s=Pascal-seconds; sec=second(s); min=minute(s); hrs=hour(s);UV=ultraviolet, and IR=infrared. All temperature references are ° C.unless otherwise specified. Ranges specified are to be read asinclusive, unless specifically identified otherwise.

The high quality plastic sheet formed by the process of the presentinvention can be used in a number of applications, including but notlimited to: substrates for electronic display devices, such as LCD andelectroluminescent displays; substrates for microoptic lens arrays andlight directing films; optical windows and filters; waveguide optics;substrates for optical, magnetic, chemical or other types of storage orrecording media; substrates for imaging, such as for photographic orx-ray applications; substrates for diagnostic systems; and substratesfor electronic circuits.

A sheet or film of the present invention is suitable for use as asubstrate of an electronic display device, for example a liquid crystaldisplay device. Such substrates are often coated with one or morecoating layers prior to applying a conductive coating, or a layer ofactive electronic devices, for example thin film transistors or diodes.The types of coatings that may be applied include crosslinked coatings,barrier coatings and conductive coatings.

A crosslinked coating layer may improve solvent resistance, abrasionresistance, and may promote adhesion between the plastic substrate and asubsequent coating layer (for example, between an organic and aninorganic coating). Crosslinked coating layers, if used, may be appliedto one or both sides of the plastic substrate.

A barrier layer is a coating which reduces gas or moisture permeation.The composition of a barrier layer may be organic, or inorganic. Abarrier coating may also be useful as a solvent resistant coating if thematerial of the barrier coating is solvent resistant and can prevent orreduce significantly the migration of such solvent(s) to the plasticsheet. Barrier layers, if used, may be applied to one or both sides ofthe plastic sheet.

The substrate of the present invention may be coated with a conductivelayer for use in optical displays. For example, if the substrate is tobe used in a liquid crystal display (LCD), an electronic component isrequired on at least one side of the substrate. Typically, theelectronic component is applied only to one side of the substrate, theside which will be “inside” the LCD cell and closest to the liquidcrystal. In the alternative, the electronic component may be applied toboth sides of the substrate. In another embodiment, one or more layersof protective coatings, color filter coatings, or barrier coatings aredisposed between the substrate and the electronic component. Suitableelectronic components include, but are not limited to a layer of activeelectronic devices or a conductive layer. Such substrates comprising alayer of active electronic devices are especially suitable for use inLCDs.

Substrates of the present invention may be incorporated into a liquidcrystal display cell by incorporating materials and processes similar tothose in W. C. O'Mara, Liquid Crystal Flat Panel Displays, Van NostrandReinhold, New York (1993). The process of forming a liquid crystal cellfrom substrates may include one or more of the following steps:patterning a clear conductive film on at least one substrate using aphotolithographic process, applying a liquid crystal alignment materialto the conductive coating on the two substrates, rubbing the alignmentlayers to impart the alignment characteristics to the substrates,applying spacer particles to at least one substrate, applying an edgeseal to at least one substrate, contacting the two substrates in theproper orientation with conductive layers facing each other, curing theedge seal, injecting liquid crystal into the narrow gap formed betweenthe substrates, and sealing the gap. The substrates of the presentinvention may be used in all types of liquid crystal display cells,including those types that incorporate a composite of liquid crystal andpolymer, those for which the display picture elements are addressedactively by electronic devices on the substrate (active matrixdisplays), and those for which the display picture elements areaddressed passively (so called passive matrix displays).

The sheet of the present invention is suitable for use as a substrate inoptical storage media. Suitable optical storage media include, but arenot limited to: compact discs, recordable compact discs, read/writecompact discs, digital versatile discs, recordable digital versatilediscs, read/write digital versatile discs, and magneto-optical discs.

Compact discs (“CD”) and digital versatile discs (“DVD”) containinformation encoded as pits and grooves on a polymer substrate. Thesequence and length of pits encode information which is read with afocused laser beam through the substrate. In the typical manufacture ofcompact discs, the pits and grooves are replicated from a stamper in amold onto the substrate. Once the substrate has been molded, it is thenmetallized by the deposition of a thin layer of reflective material,such as aluminum or gold. The substrate may then optionally be coatedwith a lacquer or resin to which ink may be applied, such as to providea label. Advances in optical data storage require the encoding ofincreased amounts of information, which decreases the spacing betweenthe pits and grooves. In order to prevent errors in the reading of suchclosely packed information (that is, high density), it is important thatthe substrate be of a sufficiently high quality that it does not deformaround the pits and grooves during manufacture or writing, in the caseof writeable discs. Such deformations cause errors in the reading of theencoded information. An advantage of the present invention is that thehigh quality optical plastic sheet produced has lower birefringence,lower surface roughness and higher dimensional stability than knowninjection molded substrates. This allows for a higher density of encodedinformation with very little deformation around the pits and grooves,provides for the writing and reading of encoded information withoutoptical distortion, and gives greater signal to noise ratios.

DVD's are generally produced in the same way as compact discs, but mayfurther have an added semireflective layer and polymer layer to encodeone or more additional layers of information on top of the first(replicated) surface. Also, DVD's may have two or more discs bondedtogether with a transparent adhesive to increase the informationcontent. Typically, DVDs comprise two to four layers.

When the sheet of the present invention is used as a substrate in anoptical storage medium, the optical storage medium comprises a) a highquality plastic sheet produced by the present method; b) a reflective orsemireflective layer disposed on at least one side of the sheet; andoptionally c) a protective layer disposed on at least one side of thesheet. The reflective or semireflective layer may be formed from anyreflective or semireflective material. When a reflective layer is used,it is preferred that the reflective layer comprise a metal, such asaluminum or gold. Such reflective layer is typically applied to thesubstrate by sputtering or vacuum deposition. When a semireflectivelayer is used, it is preferred that the semireflective layer comprises ametal. Suitable metals for use in semireflective layers include, but arenot limited to gold, aluminum, and metal alloys, such as aluminumalloys. When metals are used as the semireflective layer, only a verythin layer of the metal is required. The protective layer may be anycoating applied to the plastic substrate or reflective layer that doesnot interfere with the desired optical properties. It is preferred thatthe protective layer is a lacquer or resin. It is further preferred thatthe plastic substrate is selected from the group consisting ofhomopolymers and copolymers of polycarbonate, polystyrene, polyacrylic,polyester, polyolefin, polyacrylate, and mixtures thereof.

Information may be encoded on optical storage media comprising the sheetof the present invention by any known means, such as embossing orimaging. For example, optical storage media may be prepared by hotembossing processes, such as those described in M. T. Gale,Micro-Optics, (Ed . H. P. Herzig), Taylor & Francis, London, UK, chapter6, (1997). Hot embossing uses a heated stamper to encode information onthe substrate. The hot stamper may be used in a continuous process (rollto roll) or in a discrete process, such as compression molding. In thealternative, the substrate may first be coated with a thin polymer filmprior to embossing. Suitable polymer films include, but are not limitedto: latexes, photopolymers, and ultraviolet curable resins. Thethickness of the polymer coating is generally 0.05 to 10 microns, andpreferably 0.1 to 1 microns. Such a polymer film results in fasterembossing with better replication of the encoded information.

In the alternative, a photopolymer may be used to encode information onoptical storage media. When a photopolymer is used, it is first coatedon the substrate. The coated substrate is then imaged using anylithographic process known in the art, such as those used in thesemiconductor industry.

After the information is encoded on the substrate, the substrate iscoated with a reflective or semireflective layer, and optionally coatedwith an ultraviolet curable resin to protect the reflective orsemireflective surface. The reflective or semireflective layer may beapplied by sputtering or vacuum deposition. In the preparation of DVD's,the encoded substrate is preferably coated with a semireflective layer.The reflective or semireflective surface on the final layer is typicallycoated with an ultraviolet curable resin.

Pits used to encode information can be 0.01 to 1 microns deep andpreferably 0.04 to 0.15 micronsdeep, and 0.4 to 10 microns long. Opticalstorage media are typically up 0.6 to 1.2 millimeters thick. Thebirefringence of the substrate used in optical storage media istypically less than 50 nanometers, and preferably less than 30nanometers.

The high quality sheet produced by the present process is useful as asubstrate for optical phase-change media. Optical phase-change media arethose which can be written once and read many times and those which canbe written and read many times. Suitable optical phase-change mediainclude, but are not limited to: record once compact discs (“CD-R”),record once digital versatile discs (“DVD-R”), magnetooptical media(“MO”), and phase change media (“PD”), read/write compact discs(“CD-RW”), and read/write digital versatile discs (“DVD-RW”). Forexample, various phase-change media are disclosed in H. Bennett, EmediaProfessional, Online Inc., Wilton, Conn., July, 1998, page 31. Opticalphase-change media typically consist of a high quality substrate with aspiral groove for laser tracking purposes, a reflective orsemireflective layer, a phase-change medium disposed between thesubstrate and the reflective or semireflective layer and a UV curableprotective coating for the reflective or semireflective layer.

Information is encoded onto a phase-change medium by focusing a laserbeam on a submicron spot on the reflective layer to either causedistortions (i.e. create pits) or a phase-change (crystalline toamorphous transitions) resulting in a change in reflectivity. Incontrast, in magneto-optical (“MO”) storage media, there is a change inthe polarization of reflected light.

The spiral groove in the optical phase-change media may be prepared byembossing or imaging. For example, the high quality sheet of the presentprocess may be coated with a polymer layer and then embossed with a hotstamper to produce a spiral groove. In the alternative, a photopolymermay be applied to the high quality sheet of the present process and thegrooves defined by lithographic processes, after which the coating iscured. The polymers useful in coating the high quality optical sheet foruse in phase-change media must have a sufficiently high glass transitiontemperature to withstand subsequent high temperature deposition steps ofthe encoding process. The width and pitch of the spiral groove depend onthe construction of the particular medium but are typically in the 0.1to 10 micron range, and preferably in the 0.4 to 2 micron range. Thespot sizes of encoded information are typically in the 0.4 to 10 micronrange.

The recordable medium useful in recordable optical phase-change mediadepends upon the particular phase-change medium, and is well known tothose skilled in the art. For example, in CD-R the recordable mediumtypically consists of a dye, such as cyanine or phthalocyanine, togetherwith other polymer additives in a solvent which is spun coated onto thegrooved substrates. Any dye is suitable as long as it does not interactwith the high quality plastic substrates so as to distort the groovepattern or warp the substrate. Suitable dyes include organic dyes,inorganic pigments and mixtures thereof. The dye coating is then dried,metallized and coated with a UV curable resin to produce the final CD-R.

MO storage media comprise a high quality substrate, a magneto-opticallayer disposed on at least one side of the substrate, a reflective orsemireflective layer disposed on the magneto-optical layer andoptionally a protective layer disposed on at least one side of thesubstrate. In MO media, the magnetooptical layer, such as various alloysof cobalt, such as GdFeCo or TbFeCo, is sputtered onto the groovedsubstrates. Additional layers may be deposited on the cobalt alloy layerto aid in heat transfer to and from the substrate. Magnetic domains arethen written using a combination of a focused laser to provide localizedheating and a magnetic head to invert polarity of the domain. In CD-RWand DVD-RW, a combination of layers are deposited onto the grooved highquality plastic substrate, such as those described in M. Elphick, DataStorage, Penwell, Nashua, N.H., September, 1998, page 85. Such layerscomprise a lower protective layer, such as ZnS.SiO₂, a recordable layer,such as Ge₂Sb₂Te₅, an upper protective layer, an upper reflective metallayer and finally a protective UV curable lacquer.

The high quality sheet of the present invention is suitable for use assubstrates in storage media where the reading laser beam is reflectedand not transmitted through the substrate. In such storage media, thereading laser beam is focused onto the substrate and recording mediumusing a solid immersion lens (SIL). These lenses glide over the surfaceat a distance of a fraction of the wavelength of light, typically at adistance of 10 to 100 nanometers. In this application, the substratedoes not have to be optically transparent; however, it is essential thatthe recording medium have very low roughness. This application of nearfield optics is described in Mansfield et al., Appl. Phys. Lett., 57,1990, page 2615.

Multilayer storage media are a means of increasing data storagedensities. In such storage media, 2 or more layers are used to storedata instead of using a single layer of replicated pits as in a CD or 2to 4 layers as in a standard DVD. For example, U.S. Pat. Nos. 4,450,553,5,202,875, 5,263,011, 5,373,499 and 5,627,817 describe multilayerstorage media. In a multilayer medium, it is desirable that the totalthickness of the medium is equal to 1.2 millimeters in order for it tobe compatible with current standards. Thus, a 6 or 12 layer mediumrequires each substrate layer to have a thickness of 0.2 or 0.1millimeters, respectively. It is well known in the art that currentmethods of injection molding are incapable of producing such thinsubstrates having the required optical properties. Such thin,injection-molded substrates will be warped and have high stress andbirefringence. In contrast, the high quality sheet of the presentinvention may be used to prepare thin substrates having a very smoothsurface, low stress and low birefringence.

The methods described above for replicating pit structures on singlelayer CD can also be applied advantageously to multilayer storage media.For example, one or more polymer layers (coatings) may be applied to thesubstrate, such as by a continuous processes. Once coated, data areencoded on the sheet by embossing with a hot stamper. In thealternative, a photopolymer may coated on the substrate and the dataencoded by optical imaging (lithography). Once the data are encoded onthe substrate, a semireflective metallic or narrow bandpass dielectriclayer is applied to each substrate layer so that light will be reflectedfrom each substrate layer. The individual substrate layers are thenlaminated together using adhesive tie layers to form the desiredmultilayer storage medium.

The high quality sheet of the present invention is also useful assubstrates in multilayer rewriteable storage media. In such media, oncethe individual sheets are formed and coated, they are embossed with aspiral groove pattern. A medium, such as a phase change or MO medium, isthen sputtered onto the plastic substrate, followed by the applicationof a semireflective metallic or narrow band dielectric layer. Theindividual substrate layers are then laminated together using adhesivetie layers to form the desired multilayer storage medium.

When the high quality sheet of the present invention is used in magneticstorage media, the magnetic storage media comprise a high qualityplastic sheet produced by the present process having a magnetic alloydisposed on at least one side of the sheet. Any high quality plasticsheet of the present invention is suitable for use as a substrate inmagnetic storage media. It is preferred that the magnetic alloy beapplied to the high quality sheet by sputtering. Optionally, the highquality sheet may further comprise stiffening agents. Stiffening agentsare useful in applications where the high quality sheet requiresincreased modulus (stiffness). Suitable stiffening agents include, butare not limited to glass fibers, talc, silicon nitride, clay, andmixtures thereof. The present sheet offers the advantage of beinglighter, smoother, stiffer and tougher than known substrates formagnetic storage media, such as aluminum discs.

The high quality sheet of the present invention is also useful as asubstrate for 3-dimensional (“3-D”) optical storage media. 3-D opticalstorage media are useful in any application requiring high density datestorage, such as computer hard drives or similar data storage devices.Such 3-D optical storage media differ from conventional two-dimensionalstorage media, such as CD-ROM or magnetic storage media, in thatinformation is stored in 3-dimensional space and not just in the planeof the substrate. This results in a many fold increase of the storagecapacity from gigabyte per square-inch (per 6.45 square-cm) to terabyteper cubic-inch (per 16.39 cubic-cm). Several examples of 3-D opticalstorage media are known. These include those based on inorganicmaterials (or pigments), such as LiNbO₃, and organic materials (ordyes). Examples of organic materials useful in 3-D optical storage mediainclude photochromic dyes, which undergo reversible coloration andbleaching upon optical excitation by a polarized laser beam; side chainliquid crystal polymers, which orient in the electric field of a laserbeam causing a change in the local birefringence; amorphous polymerswith side chains containing dye molecules, where the dye molecules areattached with flexible spacers to the main chain and orient in thepolarized laser beam causing a change in the local birefringence;polymers with azo containing dye molecules in the side chain; lightsensitive protein molecules which undergo electronic transitions uponexcitation in a laser beam; photorefractive materials; andphotopolymers.

Typically, 3-D optical storage media utilize polarized light to cause alocal change in optical properties, i.e. through changes inbirefringence or absorption. A laser, preferably polarized, is typicallyused as the light source in 3-D optical storage media as such mediarequire the light source to be able to focus on a specific point,generally of submicron size. The narrow line width of a laser allows forsuch focusing on a specific point. For example, in such 3-D opticalstorage media using organic materials, a polarized laser beam causes acis-trans transition in the dye which changes the absorption spectrumand local optical density. It is therefore essential that the mediumhave very small birefringence so as not to distort the laser beam whichis used to write in the molecule. The medium must have very smallbirefringence when reading the stored information. It is wellappreciated to those experienced in the art, that the matrix must have avery small birefringence, so that the writing laser beam causes ameasurable change in local optical properties resulting in a largesignal to noise ratio (“SNR”). If the matrix had a high birefringence,then the SNR would be low and it would be very difficult to read thedata stored in 3-D space.

Thus, the high quality optical sheet of the present invention can beused to make 3-D optical storage media by dispersing in such sheet oneor more pigments, dyes or mixtures thereof. It is preferred that thepigments, dyes or mixtures thereof is selected from LiNbO₃, photochromicdyes, side chain liquid crystal polymers, amorphous polymers with sidechains containing dye molecules, polymers with azo containing dyemolecules in the side chain; light sensitive protein molecules,photorefractive materials; and photopolymers. It is also possible toblend in any molecule, polymer, pigment or other inorganic material thatundergoes an electronic ransition to change its absorbance, anorientation in the electric field of a polarized aser beam resulting ina change in the local refractive index, or a chemical reaction, uch ascrosslinking, to cause a change in refractive index.

Typically, the dye, pigment or mixture thereof is incorporated into theoptical sheets of the present invention by combining the dye, pigment ormixture thereof with the molten plastic resin prior to shaping themolten resin into a web using an overflow die.

The sheet produced by the present process is suitable for use in circuitboard manufacture, particularly for use as a substrate for circuitlayers. Multilayer printed circuit boards are used for a variety ofapplications and provide notable advantages of conservation of weightand space. A multilayer board is comprised of two or more circuitlayers, each circuit layer separated from another by one or more layersof dielectric material. Circuit layers are formed by applying a copperlayer onto a polymeric substrate. Printed circuits are then formed onthe copper layers by techniques well known in the art, for example printand etch to define and produce circuit traces.

After lamination, the multiple circuit layers are connected by drillingthrough-holes through the board surface. Resin smear from through-holedrilling is removed under rather stringent conditions, for exampletreatment with concentrated sulfuric acid or hot alkaline permanganate,and then through-holes are further processed and plated to provide aconductive interconnecting surface. Prior to lamination, the circuitlayers are typically treated with an adhesion promoter to improve bondstrength between each circuit layer and the interleaving resin layers.One favored method of improving such bond strength is oxidativetreatment of a circuit layer to form copper oxide surface coatingthereon.

When the sheet produced by the process of the present invention is usedas a substrate for microoptic lens arrays, the arrays may be produced byany suitable means for producing optical storage media. Suitablemicrooptic lens arrays include, but not limited to, rectilinear prisms,two dimensional arrays of pyramids, diffractive lenses and holographiclens arrays. For example, a hot embosser may be used to emboss an arrayof lenses on the sheet of the present invention. It is preferred thatthe hot embosser is used in a compression molding press. It is preferredthat the sheet of the present invention is selected from the groupconsisting of homopolymers and copolymers of polycarbonate, polystyrene,polyacrylic, polyester, polyolefin, polyacrylate, and mixtures thereof.Such microoptic lens arrays are useful in display and imagingapplications requiring the refraction or diffraction of light. Suchapplications include, but are not limited to: heads-up displays; largescreen displays; and as lens arrays in office equipment, such asphotocopiers and facsimile machines. Lenses used in microoptic arraysare typically 10 to 100 microns. In holographic lenses, the height istypically a fraction of the wavelength of light and the lateraldimensions are typically 10 to 100 microns.

When the optical quality sheet of the present invention has a structuredsurface, such sheet is suitable for use as a light management or lightdirecting film. Light directing films have transmission properties thatcontrol, and are governed by, the angle of incidence of light. Forexample, light directing films consist of a polymeric film with arectilinear periodic array of angled prisms embossed on one surface,i.e. a structured surface. The light directing film acts as a lightcollimator accepting light from the light source through its flatsurface, and transmitting it through its structured face. Light which isincident upon a smooth surface of this film at relatively high incidenceangles is refracted at the smooth surface and the structured surface ofthe film and is emitted from the sheet, having been redirected towardthe normal to the smooth surface of the film. Additionally, light whichstrikes the structured surface internally at greater than the criticalangle undergoes total internal reflection from both side surfaces, orfacets, of a prism element and is directed back toward the light source.However, when light is incident upon the structured side of the film andsubsequently transmitted to the flat surface of the film, thecomplementary behavior is observed. Namely, that light incident atangles close to the normal plane of the light directing film aretransmitted, and those light rays incident at higher angles arereflected back via total internal reflection. Such light management orlight directing films are particularly suitable for use in LCDillumination systems, such as backlight illumination.

Typically, a structured surface is provided to the optical sheets of thepresent invention by embossing the structures on the surface of thesheet while it is still warm, or on a cooled sheet by using a hotembosser. It is preferred that the structured surface be provided bycoating the optical sheet of the present invention with a curableprepolymer having suitable viscosity, embossing the desired structuredsurface on the prepolymer coating, and then curing the prepolymer. It ispreferred that the prepolymer be UV-curable. Prepolymers that can coatthe optical sheet of the present invention and that retain any embossedstructure during curing have suitable viscosity for use in lightmanagement films.

The sheet produced by the present invention is also suitable for use inwaveguide optics. Suitable waveguide optics include, but are not limitedto: waveguides, active and passive photonic switches, wavelengthdivision multiplexers, electroluminescent light sources, andelectrooptic modulators. When used in these applications, the waveguideoptics may be embossed on the sheet in the same way as microopticarrays. The advantage to waveguides fabricated from the sheets of thepresent invention is that they may be fabricated in large sheets and theexpense and difficulty of known methods is avoided. In waveguide opticapplications, it is preferred that the sheet is selected from the groupconsisting of homopolymers and copolymers of polycarbonate, polystyrene,polyacrylic, polyester, polyolefin, polyacrylate, and mixtures thereof.

The sheet of the present invention may also be used in the manufactureof microfluidic devices. Such devices include, but are not limited to:miniature diagnostic systems for biopharmaceutical applications,miniature devices for directing fluid flow, miniature sensor devices forpharmaceutical and biochemical applications, and three-dimensionalmicrofluidic systems. When used in these applications, it is preferredthat the sheet is selected from the group consisting of homopolymers andcopolymers of polycarbonate, polystyrene, polyacrylic, polyester,polyolefin, polyacrylate, and mixtures thereof.

Depending on the particular use for sheet produced by the method of thepresent invention, sheet characteristics such as low shrinkage, lowbirefringence, and surface quality may vary in relative importance.Desired sheet thickness will also vary depending on the particular use,but will generally be about 25 mm or less, preferably 10-5000μ, and mostpreferably 50-1000μ. Sheet thickness can be adjusted by varying thespeed of delivery of the molten polymer to the die or by varying thespeed of the take-off means. Thickness variation over a sample length of400 mm should be generally 10% or less, preferably 5% or less, and mostpreferably 1% or less.

A typical apparatus of the present invention is shown in FIGS. 1-4. Aswill become clear to those skilled in the art, variations from theapparatus illustrated in these Figures may be made within the scope ofthe present invention.

Molten polymer from a source 10 is delivered to an overflow die 20 viachannel 12 (preferably controlled by delivery means 14), where it isintroduced to the die 20 through conduit opening 21 to conduit 22. Thetemperature of the molten polymer as it is delivered to die 20 ismaintained by use of heaters 15 located in close proximity to die 20. Asthe molten polymer fills the opening 21, it is forced out through themetering arrangement, slot 23, onto the die lips 40 and 41, and flowsout around the sides 24 and 25 of the die 20. At the apex 26 of the die20, the molten polymer flowing from sides 24 and 25 converge to form thebeginning of molten web 27.

The molten web 27 is picked up at its edges by two pairs of guidancemeans, (e.g., tank treads 31, 32, 33 and 34) which guide the molten webaway from die 20. As molten web 27 is guided away from die 20, thetemperature of the web gradually falls below the glass transitiontemperature of the polymer, and results in cooled sheet 40. In anoptional embodiment, cooling means 36 located in close proximity to theguidance means 31, 32, 33, 34 aid in lowering the temperature of theweb.

Molten resin can be supplied in any of a number of ways. For example,the molten resin may be supplied from a polymerization reactor, a mixer,a devolatilization device (e.g., a flash column, falling stranddevolatilizer or wiped film evaporator), or an extruder. An extruder ispreferred, as it can also act as a polymer delivery means (seediscussion below). It is most preferred to use a single screw extruder,although a double (twin) screw extruder or a multiple screw extruder mayalso be used. If a twin or multiple screw extruder is used, it can be ofany type, for example, counter-rotating, co-rotating, intermeshing ornon-intermeshing. It will be appreciated that well-known techniques forhandling or preparing resins can be used in the present process. Suchtechniques include drying, use of inert atmospheres, pellet dedusting,and the like.

The molten resin may contain one or more plastic additives such asantioxidants, ultra-violet (‘UV’) absorbers, UV stabilizers, fluorescentor absorbing dyes, anti-static additives, release agents, fillers andparticulates. The type and amount of additive used with particularresins for particular purposes is known to those skilled in the plasticarts and will not be further detailed herein.

The temperature at which the resin is processed will depend upon thecomposition of the resin and may vary during processing. The temperaturemust be sufficiently high that the resin will flow but not so high as todegrade the resin. Operating conditions will vary depending on the typeof polymer to be processed, and are within ranges known to those skilledin the art. However, as a general guideline, the operating temperaturewill be between 100 and 400° C. For example, PMMA may be processed in anextruder with the extruder barrel temperature of 150 to 260° C. and amelt temperature of 150 to 260° C. Other polymers such as polycarbonateor poly methylmethacrylimide can also be used at appropriately highermelt temperatures (200-330° C.). It is preferred that volatile materialsand undesired particulate matter be removed from the molten plasticresin prior to sheet formation. This may be accomplished in accordancewith methods known to those skilled in the art.

Delivery means 14 for delivering constant flow of the molten polymer arerequired for the purpose of regulating the flow rate and providing thepressure required to deliver the molten polymer through the channel 12,conduit opening 21 and conduit 22, to the die 20. The delivery means mayinclude any type of mechanical melt pump, including, but not limited toany appropriate extruder (as described above), gear pump, orcombinations thereof. In simple form, the delivery means may be agravity feed, or hydrostatic pressure. The delivery means may beselected in accordance with methods known to those skilled in the art.The use of a gear-type melt pump is preferred because it providescontrol of flow rate and minimizes flow rate fluctuations, resulting inmore uniform sheet thickness. In addition, the use of a melt pump mayreduce degradation of the molten resin by reducing the shear heating ofthe polymer. Temperatures for the melt pump are determined by theplastic resin used, and are similar to those used in standard extrusionprocesses, typically between 50 and 200° C. above the Tg of the resin.More than one delivery means may be used, for example, for theproduction of wide sheets. In the present invention, the delivery meansshould provide molten polymer to the inlet of the overflow die in therange of 50 to 70,000 kpa, preferably 300 to 7000 kPa, and mostpreferably 1000 to 3500 kPa.

In one embodiment, the polymer melt is passed through a melt filter or amixer between the delivery means 14 and the die 20. It is preferred thatthe polymer melt is passed through a melt filter and then a mixer. Thefilter removes gels, dirt and foreign particles from the melt. The mixerblends the polymer in order to minimize thermal gradients in the meltand removes flow lines resulting from the melt filter. Any melt filtermay be used in the present process. Suitable melt filters include candlefilters and disc filters. It is preferred that the filter is a discfilter. Any mixer may be used in the present process, such as staticmixers or rotary mixers. The use of a melt filter and mixer producesplastic sheet having better quality in terms of smoothness and thicknesscontrol.

The overflow die is used to form a sheet from the molten plastic resin.The die includes a metering arrangement and an overflow surface withconverging sides which in cross section culminate in an apex. The die inlengthwise fashion can be substantially linear, curved, oval orcircular. The die height to width ratio should generally be in the rangeof 1:1 to 10:1, preferably 2:1 to 5:1, and most preferably 2.5:1 to 4:1.The length (or circumference) to height ratio should generally be atleast 1:2, preferably at least 2:1, and most preferably at least 3:1.

The metering arrangement portion of the overflow die consists of flowdistribution elements such as, for example, holes, slot, “coat hanger”arrangement or combinations thereof, which control the flow distributionof the molten resin across the die, thereby controlling the sheetthickness profile. Examples of such metering arrangements areillustrated in FIGS. 5-7. Other metering arrangements may be used asknown to those skilled in the art. A slot arrangement is preferred. Thelength of the die will depend upon the width of the sheet to be made,but the ratio of the mean slot gap (mean width of the slot 23) to meanconduit diameter (mean diameter of the conduit 22) should generally beat least 1:5, preferably at least 1:10, and most preferably at least1:20. For sheets having a finished thickness of 1 mm or less, asubstantially constant slot width across the die is preferred. Forgreater thicknesses, a tapered slot is preferred wherein the slot isthinner at the feed end, and thicker at the opposing end. If a widesheet is desired, conduit openings 21 and 21′ (see FIG. 6) can belocated at both ends of the die, and it is possible to have the slot 23tapered at both ends.

The overflow surface is formed by the exterior of the die 20 andconsists of a pair of die lips, 40 and 41, which connect with themetering arrangement and direct the molten polymer to the convergingsides, 24 and 25. The converging sides direct the melt flow to the apex26, where the melt web exits from the die. Although the overflow surfacecan be textured or smooth, it is preferably smooth. Moreover, theoverflow surface is preferably highly polished to minimize variationsand defects in the sheet. The overflow surface may be treated with acoating (for example, electroplating or other depositing techniques) toimprove die surface smoothness, provide corrosion resistance, or improvethe flow properties over the die.

The material of construction of the die is important. Metals arepreferred due to their high thermal conductivity, good corrosionresistance, high modulus, and ability to be polished. However, othermaterials such as glass and ceramics can, in principle, be used. It ispreferred to use stainless or tool grade steel.

If a non-planar sheet is desired, the die geometry may be modifiedaccordingly, using methods known to those skilled in the art. Forexample, if a curved sheet is desired, the die can be curved along itslongitudinal axis.

In general, it is desired to maintain the viscosity of the moltenplastic (for a shear rate of 10 sec⁻¹) between 1 and 10,000 Pa-s,preferably between 5 and 1,000 Pa-s, and most preferably between 10 and500 Pa-s. In addition, the melt flow rate per unit die length (flow ratedivided by the length) is typically in the range of 1.0×10⁻³ to 10g/s/cm, preferably 1.0×10⁻² to 1.0 g/s/cm, and most preferably 2.0×10⁻²to 2.0×10⁻¹ g/s/cm. The viscosity can be controlled by varying thetemperature. Depending on the die design, the temperature control may bemore or less important. The more even the temperature across the die,the more even the thickness of the sheet. It is preferred that the melttemperature be uniform across the die. Thickness variation resultingfrom uneven temperature distribution down the length of the die can beminimized by changing the design of the slot or other meteringarrangement. Temperature control may be accomplished, for example, byone or more of the following: electric cartridge heaters, infrared lampheaters, heated oil (or other heat transfer fluid), heat pipes, ormicrowave heaters. Heated oil or other heat transfer fluids arepreferred because the temperature may be controlled by a thermostat anduniformity of temperature may be readily accomplished. The die ispreferably housed within a partially enclosed area in order to minimizetemperature fluctuations. It is preferred that an inert environment alsobe used. Such inert environment minimizes coloration and degradation ofthe resin.

It is preferred, but not essential, that the molten plastic flows in adownward direction after passing over the die, since the downward flowis affected by gravity. The rate of flow is determined by a combinationof the effect of gravity, and the tension applied by the takeoff means.By conducting the plastic flow in a downward direction over the die,gravity acts in the same direction as the sheet flow, thereby reducingthe tension needed in the takeoff means and improving sheet quality. Themolten plastic after passing through the die is in a form known as a“web.”

The takeoff means transports the molten plastic web from the die at acontrolled speed and allows the web to cool. The takeoff means may be,for example, rollers or a “tank tread” arrangement, whereby only theouter edges of the sheet come into contact with the takeoff means. A“tank tread” arrangement is preferred, as this maximizes the smoothnessof the sheet surface. A tank tread arrangement is illustrated as part ofthe apparatus of FIGS. 1 and 2 as 31, 32, 33 and 34.

The takeoff means controls the speed at which the plastic sheet isproduced, which at a given polymer flow rate determines the thickness ofthe sheet; therefore, control of the speed of the takeoff means is quiteimportant. The takeoff means also supports the weight of the sheet,thereby maintaining consistent sheet width and thickness. It isdesirable to position the takeoff means as close as possible to the dieso that the amount of molten resin that is unsupported is minimized. Thedistance from the apex of the die to the takeoff system (e.g., the niparea at the top of the tank tread arrangement) is typically <25 cm,preferably <10 cm, and most preferably <5 cm.

The sheet takeoff speed will vary depending on the type of sheetdesired, and the thickness. For example, for a sheet having 0.4 mmthickness, the sheet takeoff speed will generally be in the range of 10to 1000 cm/min, preferably 20 to 200 cm/min, and most preferably 50 to100 cm/min; whereas for a sheet having 1 mm thickness, the takeoff speedwill generally be in the range of 5 to 500 cm/min, preferably 10 to 100cm/min, and most preferably 25 to 50 cm/min. In like fashion, theresidence time during cooling in the takeoff system before bending willvary. For example, for a sheet having 0.4 mm thickness, the residencetime before bending will generally be ≧10 sec, preferably ≧1 min, andmost preferably ≧2 min; whereas for a sheet having 0.2 mm thickness, theresidence time before bending will generally be ≧5 sec, preferably ≧30sec, and most preferably ≧1 min.

The plastic sheet may be allowed to cool by natural convection duringtransport by the takeoff system, or by forced convection. Naturalconvection consists of passive cooling of the sheet during passagethrough air or a fluid bath. Forced convection is accomplished bypumping or blowing a heat transfer fluid along or against the sheet toenhance heat transfer. Natural convection is preferred for minimizingsheet ripples and surface marks. It is preferred to use a clean fluid(free from particulates) for cooling the sheet to prevent surfacecontamination or defects. For example, HEPA filters may be used with airor gas cooling for this purpose. Any fluid or combinations of fluids canbe used for sheet cooling, provided that the fluid used is notdetrimental to the plastic material being processed. Examples of usefulcooling fluids are: air, nitrogen, water, oils, and glycols. It ispossible to combine the cooling process with a coating process by usinga suitable coolant which acts as a coating and is deposited as a film onthe plastic sheet as it leaves the cooling bath.

It will be recognized by those skilled in the art that a variety ofoptional equipment may be used following the takeoff means. Examples ofoptional equipment include conventional film handling equipment such asfilm winders, edge cutters, sheet cutters, and packaging equipment. Inaddition, other downstream devices can be utilized, for example, formingequipment, coating equipment, decorating equipment, and laminatingequipment.

The process of the present invention may be used with any suitableplastic resin, and is preferably used with thermoplastic resins. Athermoplastic resin is a polymeric resin which reversibly softens whenexposed to heat and hardens upon cooling. Thermoplastic resins may belinear or branched polymers that are not substantially cross-linked. Itis preferred that the thermoplastic resins useful in the process of thepresent invention have virtually no crosslinking and have thermalstability (for residence time of up to 10 min or more) at meltprocessing temperatures (i.e., having a viscosity on the order of 10³Pa-s). Examples of thermoplastic resins for which the process of thepresent invention is useful include but are not limited to: homopolymersor copolymers of acrylic acid, methacrylic acid and their esters,including but not limited to copolymers formed with styrene and itsderivatives, N-alkyl maleimides, acrylonitrile, and vinyl acetate;phenoxy ethers; polyphenylene oxide resins, epoxy resins; cellulosicresins; vinyl polymers such as polyvinyl chloride (“PVC”);fluoropolymers such as fluorinated ethylene-propylene andpoly(vinylidene fluoride); polystyrenes; polyolefins such aspolyethylene, polypropylene, poly-4-methylpentene-1, and includingcyclic olefin polymers and copolymers, such as those based on norborneneand functionalized norbornene monomers; polysulfones; polyethersulfones; polyether ketones; polyether imides; polyphenylene sulfides;polyarylene ester resins; polyesters; homopolymers or copolymers of N—Hand/or N-alkyl glutarimide; acrylonitrile-butadiene-styrene resins(“ABS”); styrene-acrylonitrile resins (“SAN”); styrene-maleic anhydrideresins (“SMA”); imidized SMA; polyamides (“Nylons”); polycarbonates,including high temperature homopolymers and copolymers;polycarbonate-polyesters; polyarylates; liquid crystal polymers; andmixtures thereof. Suitable polycarbonates comprise one or morebisphenols and one or more carbonic acids. Suitable carbonic acidsinclude, but are not limited to: phosgene, diphosgene, triphosgene,carbonic acid esters, such as chloroformic acid esters, and mixturesthereof. Suitable bisphenols include, but are not limited to:bis(4-hydroxyphenyl) alkanes and cycloalkanes;bis(3-substituted-4-hydroxyphenyl) alkyl-cycloalkanes;bis(3,5-disubstituted-4-hydroxyphenyl) alkylcycloalkanes, such as2,2-bis(4-hydroxyphenyl)propane;2,2-bis(3-methyl-4-hydroxyphenyl)propane;2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane;1,1-bis(4-hydroxyphenyl)cyclohexane;2,2-bis(3-phenyl-4hydroxyphenyl)propane;2,2-bis(3isopropyl-4-hydroxyphenyl)propane;2,2-bis(4-hydroxyphenyl)butane; 9,9-bis(4-hydroxyphenyl)fluorene;9,9-bis(4-hydroxy-3-methylphenyl)fluorene;1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4hydroxyphenyl)-3,3-dimethylcyclohexane;1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,3-bis(4-hydroxyphenyl)-5,7-dimethyladamantane;1,1-bis(4-hydroxyphenyl)-1-phenylethane;4,4′-dihydroxy-tetraphenylmethane; 2,2-bis(4-hydroxyphenyl)propane;6,6′-dihydroxy-3,3,3′,3′-tetramethyl-1,1′-spiro(bis)indane; and mixturesthereof. The polyester-polycarbonates useful in the present inventioncomprise one or more bisphenols, one or more carbonic acids, and one ormore additional acids, such as terephthalic and isophthalic acids. Thepolycarbonates and polyester-polycarbonates useful in the presentinvention are well known in the art. Mixtures of thermoplastic resinsmay also be used. Particularly useful thermoplastic resin mixturesinclude, for example: SAN-polyglutarimide, polycarbonate-polyester,PMMA-poly(vinylidene fluoride), polystyrene-poly(phenylene oxide), andpolycarbonate blends, including blends bisphenol A polycarbonate andhigh temperature polycarbonate copolymers, such as a copolymer of2,2-bis(4-hydroxyphenyl)propane and1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, available as APECpolycarbonate (Bayer Corp.). Preferred resins for use in the process andapparatus of the present invention are: polycarbonates; linear acrylichomopolymers and copolymers; cyclic polyolefins; and linear imidizedacrylic homopolymers and copolymers such as those described in U.S. Pat.No. 4,727,117 (Hallden-Abberton et al.) and U.S. Pat. No. 4,246,374(Kopchik).

The plastic resins useful in the present invention typically result fromaddition polymerization or condensation polymerization processes.Addition polymerization processes include bulk polymerization andsolution or dispersion polymerization in water or organic solvent media;such processes are well known in the art and may incorporate cationic,anionic, or free radical initiation and propagation reactions.Condensation polymerization processes include bulk, solution anddispersion polymerization processes. Plastic resins formed bypolymerization processes other than bulk polymerization may requiresubsequent treatment in order to isolate the resin.

The following examples are presented to illustrate further variousaspects of the present invention, but are not intended to limit thescope of the invention in any respect.

EXAMPLE 1 Preparation of Acrylic Film

This example illustrates the method of the present invention used toproduce optical quality acrylic sheet.

PMMA resin having an average molecular weight of 110,000 was starve-fedinto a 2 inch (5 cm) diameter single screw vented two-stage extruderhaving a 30:1 L:D ratio at a rate of 3.1 g/s using a volumetric feeder.The extruder barrel had a temperature profile from 204° C. at the feedend to 274° C. at the discharge end. The resin was devolatilized using adevolatilization vent operating at 720-750 mm Hg. The screw was rotatedat 30 rpm. A gear-type melt pump was used to pump the molten resinthrough a screen pack filter to a 12″ (30 cm) long overflow die having a1.27 cm diameter internal conduit and a series of 22 metering holes witha spacing of 1.27 cm. The diameter of the metering holes increased fromthe feed end of the die to the downstream end from 3.18 mm to 3.73 mm.The melt pump temperature was 274° C. The melt pump suction pressure was2100 kPa. and the melt pump discharge pressure was approximately 4100kPa. The overflow die was heated internally using three electriccartridge heaters and externally using three IR heating units to atemperature of 274° C. The molten web formed at the apex of the die wasconveyed using two pairs of tank treads, and cooled using cooled forcedair which was applied using two air plenums.

The resultant sheet had average thickness of 0.325 mm, surface roughnessRq of 14.6 nm and an optical retardance of <5 nm.

EXAMPLE 2 Preparation of Imidized Acrylic Sheet

This example illustrates the method of the present invention used toproduce optical quality imidized acrylic sheet.

A capped imidized acrylic resin having an weight average molecularweight of 108,000 and a glass transition temperature of about 180° C.was starve-fed into a 2 inch (5 cm) diameter single screw ventedtwo-stage extruder having a 30:1 L:D ratio at a rate of 2.5 g/s using agravimetric feeder. The extruder barrel had a temperature profile from246° C. at the feed end to 329° C. at the discharge end. The resin wasdevolatilized using a devolatilization vent operating at 720-750 mm Hg.The screw was rotated at 30 rpm. A gear-type melt pump was used to pumpthe molten resin through a screen pack filter to a 25.5 inch (65 cm)long overflow die with a 1.588 cm diameter internal conduit and a 16inch (40 cm) long slot tapering from 0.038 to 0.042 inch (0.965 to 1.067mm). The melt pump temperature was 329° C. The melt pump suctionpressure was approximately 4100 kPa. The melt pump discharge pressurewas approximately 1650 kPa. The die was heated using a hot oil system(oil temperature=343° C.) via internal holes in the die, and the airaround the die was heated with a forced-air oven (temperature=280° C.).The molten web formed at the apex of the die was conveyed using twopairs of tank treads operating at a speed of 1.2 cm/s, and cooled bynatural convection of room air.

A 200 mm×200 mm piece was cut from the cooled sheet and tested. Theresultant sheet had a thickness of 0.390 mm, with a variation of ±0.015mm. The surface waviness Wy and Wq were <0.5μ and 0.18μ respectively,surface roughness Rq was 7.6 NE, and the optical retardance was <6 NE.The thermal shrinkage, measured at a temperature of 160° C., was 0.03%or less.

EXAMPLE 3 Preparation of Polycarbonate Sheet

This example illustrates the method of the present invention used toproduce optical quality polycarbonate sheet.

Extrusion-grade polycarbonate resin (Lexan 101, GE Plastics, Pittsfield,Mass.) was starve-fed into a 2 inch (5 cm) diameter single screw ventedtwo-stage extruder having a 30:1 L:D ratio at a rate of 4.4 g/s using agravimetric feeder. The extruder barrel had a temperature profile from232° C. at the feed end to 315° C. at the discharge end. The resin wasdevolatilized using a devolatilization vent operating at 720-750 mm Hg.The screw was rotated at 30 rpm. A gear-type melt pump was used to pumpthe molten resin through a screen pack filter to a 37.5 inch (95 cm)long overflow die with a 1.905 cm diameter internal conduit and a 28inch (71 cm) long slot tapering from 0.038 to 0.045 inch (0.965 to 1.143mm). The melt pump temperature was 315° C. The melt pump suctionpressure was approximately 3400 kPa. The melt pump discharge pressurewas approximately 1300 kPa. The die was heated using a hot oil system(oil temperature=315° C.) via internal holes in the die, and the airaround the die was heated with a forced-air oven (temperature=260° C.).The molten web formed at the apex of the die was conveyed using twopairs of tank treads operating at a speed of 1.2 cm/s, and cooled bynatural convection of room air.

A 400 mm×400 min piece was cut from the cooled sheet and tested. Theresultant sheet had an average thickness of 0.43 mm, with a variation of±0.02 mm in both the transverse and machine directions. Wy was <1μ, Wqwas 0.15μ, the surface roughness Rq was <10 nm, and the average opticalretardance was 20 nm with a variation of 10 nm. Thermal shrinkage,measured at 130° C., was 0.02%.

EXAMPLE 4 Preparation of Polycarbonate Film

This example illustrates the method of the present invention used toproduce optical quality polycarbonate film.

Extrusion-grade polycarbonate resin (Lexan 101, GE Plastics, Pittsfield,Mass.) was starve-fed into a 2 inch (5 cm) diameter single screw ventedtwo-stage extruder having a 30:1 L:D ratio at a rate of 2.5 g/s using agravimetric feeder. The extruder barrel had a temperature profile from232° C. at the feed end to 315° C. at the discharge end. The resin wasdevolatilized using a devolatilization vent operating at 720-750 mm Hg.The screw was rotated at 30 rpm. A gear-type melt pump was used to pumpthe molten resin through a screen pack filter to a 37.5 inch (95 cm)long overflow die with a 1.905 cm diameter internal conduit and a 28inch (71 cm) long slot tapering from 0.038 to 0.045 inch (0.965 to 1.143mm). The melt pump temperature was 315° C. The melt pump suctionpressure was approximately 3400 kPa. The melt pump discharge pressurewas approximately 1300 kPa. The die was heated using a hot oil system(oil temperature=315° C.) via internal holes in the die, and the airaround the die was heated with a forced-air oven (temperature=250° C.).The molten web formed at the apex of the die was conveyed using twopairs of tank treads operating at a speed of 3.1 cm/s, and cooled bynatural convection of room air.

A 400 mm×400 mm piece was cut from the cooled sheet and tested. Theresultant film had an average thickness of 54μ, with variation ±4μ inboth the transverse and machine directions, and an optical retardance of<10 nm.

EXAMPLE 5 Preparation of High Temperature Polycarbonate Blend Sheet

A pre-compounded blend of 2.33 parts APEC DP9-9371 a polycarbonatecopolymer available from Bayer, Corp., Pittsburgh, Pa., with a Tg of205° C. to 1 part Makrolon DP-1265 a low molecular weight polycarbonate,also available from Bayer, Corp., with a Tg of 150° C. was starve fedinto an extruder at a rate of 2.5 g/s using a gravimetric feeder. Theextruder was a 5.08 cm (2 inch) diameter two stage single screw ventedextruder having a 30:1 L:D ratio, a 10 cc/rev gear-type melt pump, meltfilter, rotary mixer and overflow die. All process equipment was purgedwith nitrogen gas prior to start-up. The resin was devolatilized using adevolatization vent operating at 720-750 mm Hg. The screw was rotated at30 RPM. The gear pump was used to meter the molten resin through a 5micron sintered metal fiber melt filter (pleated candle type). The fluxof resin across the filter was 8.6 lb./hr./sq.ft. at a pressure drop of3800 kPa. The extruder barrel had a temperature profile from 273° C. atthe feed end to 304° C. at the discharge. The melt filter was maintainedat 322° C. The extruder feed hopper and oven around the overflow diewere inerted with nitrogen to minimize the formation of crosslinked gelsin the resin. After exiting the melt filter, the resin entered a rotarymixer where the molten polymer was mixed at 165 RPM. The mixer barreltemperature was maintained at 325° C. The molten resin exited the mixerand entered the 37.5″ (95 cm) long overflow die with a 1.905 cm diameterinternal conduit and a 28 inch (71 cm) long slot tapering from 0.038 to0.045 inch (0.965 to 1.143 mm). The die entrance pressure was about 700kPa and was maintained at a temperature of 321° C. by circulating hotoil through internal passages. The nitrogen around the die was heated to240° C. with a forced convection oven. The molten web formed at the apexof the die was conveyed using pairs of tank treads operating at a speedof 0.7 cm/s and cooled by natural convection of room air.

A 400 mm×400 mm piece was cut from the cooled sheet and tested. Theresultant sheet had a thickness of 0.420 mm, with a variation of+/−0.020 mm. Gel counts were less than 200/sq. meter. The surfacewaviness Wy and Wq were 1 μm and 0.17 μm respectively. Surface roughnessRq was <10 nm. The optical retardance was <20 nm. The thermal shrinkagemeasured at a temperature of 160° C. was <0.05%.

EXAMPLES 6-8 Fabrication of Optical Storage Media

Three 1.2 mm polycarbonate sheets are prepared according to Example 3.To each sheet is applied a polymer coating as described below. Once thesheet is coated, it is then embossed using a hot stamper. The stampercontains bumps to create submicron pits in the plastic sheet. The hotstamper is applied to each of the sheets with pressure for a fewseconds. After embossing, the sheets are then coated with a reflectivemetal layer using a sputtering coater. The metal layer on each sheet isthen coated with a UV curable polyacrylate. The sheets are then cut in apress to provide 120 mm discs suitable for use in optical storage media.

Example # Polymer Coating 6 Euderm 50UD 7 S1828 8 Norland 61

Euderm 50UD is a latex coating, available from the Rohm and HaasCompany, Philadelphia, Pa.

S1828 is a photopolymer, available from Shipley Company, Marlborough,Mass.

Norland 61 is a UV curable resin, available from Norland Company, NewBrunswick, N.J.

Test Methods

The following test methods were used to test the sheets made in theExamples above. It is understood in the art that these test methods areexemplary in nature, and that the results are not method-dependent.

A. Optical retardance

The retardance of light at 632.8 nm wavelength was determined in thefollowing manner. A polarized laser beam (polarized at −45° with respectto the laboratory frame) was passed through the plastic sheet, and thenthrough a photoelastic modulator (PEM) (Model PEM-90, Hinds Instruments,Inc.; Hillsboro, Oreg.) oriented with optical axis set to 0° in the labframe. The PEM voltage was set at ¼ wave retardance (158.2 nm). Thelight then was passed through a second linear polarizer (polarizationaxis +45°) and intensity detected by a silicon diode detector (ModelPDA-50, ThorLabs Inc.; Newton, N.J.). The PEM and detector weremodulated, and the signal from the detector processed by a lock-inamplifier (Model 5210, E G & G Princeton Applied Research; Princeton,N.J.). The plastic sheet was rotated perpendicular to the laser beam tofind the maximum signal. The retardance was determined by comparing themaximum signal to that measured for a standard ¼ wave plate.

Birefringence of a material can be obtained by dividing the opticalretardance of a material by its thickness. For example, if the opticalretardance for a 0.4 mm thick sheet of plastic is 4 nm, thebirefringence of the materials is 0.00001. For optical quality plasticsheet made by the method of the present invention, birefringence of amaterial is considered to be low if it is ≦0.0002, preferably ≦0.00005,and most preferably ≦0.00001.

B. Sheet waviness

Sheet waviness (Wy and Wq) was measured using a stylus profiler(Surfanalyzer System 5000, Federal Products; Providence, R.I.) with aprocedure similar to that of SEMI Standard D15-1296. The measuredprofile was digitally filtered with a Gaussian long wavelength cutoff (8mm). Wy is the difference between maximum and minimum values in an 20 mmsampling length, and Wq is the root mean square average deviation of thefiltered profile from the mean line calculated over 8 mm, and averagedover a 80 mm evaluation length. For high quality sheet produced by themethod of the present invention, Wy should be ≦2.0μ, preferably ≦1.0μ,and most preferably ≦0.5μ.

C. Sheet roughness

Sheet roughness (Rq) was measured using a stylus profiler (Dektak 3-30,Veeco/Sloan; Santa Barbara, Calif.) with a procedure similar to that ofSEMI Standard D7-94. The measured profile was digitally filtered with aGaussian long wavelength cutoff (0.08 mm) and a short wavelength cutoff(0.0025 mm). The evaluation length was 0.4 mm. The roughness parameter(Rq) is the root mean square average deviation of the filtered profilefrom a mean line. The average value from three different measurementswas reported. For high quality sheet produced by the method of thepresent invention, Rq should be ≦50 nm, preferably ≦10 nm, and mostpreferably <5 nm.

D. Shrinkage

Shrinkage was determined by directly measuring the sample length beforeand after heat treatment. Multiple measurements were made to determinethe length of a dry piece of plastic. The accuracy of the measurementwas 0.005%. The sample was heated to a set temperature below its T_(g)for 4 hours. Upon cooling to room temperature, the length was againdetermined by multiple measurements. The percentage change in lengthbefore and after the heating cycle was reported as the shrinkage. Forhigh quality sheet produced by the method of the present invention, theshrinkage should be ≦0.10%, preferably ≦0.075%, and most preferably≦0.05%.

Gel Counting Test Method:

Gels counts were measured by projection of sheet defects onto a screenby means of an overhead projector. Nine 3×3 cm areas on each 400 mm×400mm sheet were subject to counting. The number of surface defectsprojected onto the screen within each of the 3×3 cm squares was counted,and the numbers averaged to give a total gel count for the sample.

What is claimed is:
 1. A 3-dimensional optical storage medium comprisinga high quality plastic sheet having dispersed thereon one or morepigments, dyes or mixtures thereof, wherein the pigments, dyes ormixtures thereof undergo a change in optical properties locally uponexposure to light; wherein the optical storage medium is produced by thesteps of: a) providing molten plastic resin; b) directing the moltenplastic resin to an overflow die having an inlet and an outlet; c)shaping the molten plastic resin into a molten web using said overflowdie; d) combining the pigment, dye or mixture thereof with the moltenplastic resin prior to shaping the molten resin into a web using saidoverflow die; e) guiding said molten web away from said overflow die;and f) cooling said molten web to form a solid; wherein the high qualityplastic sheet has a birefringence of less than or equal to 0.0002. 2.The 3-dimensional optical storage medium of claim 1 wherein the resin isa thermoplastic resin selected from the group consisting of:homopolymers or copolymers of acrylic acid, methacrylic acid and theiresters; phenoxy ethers; polyphenylene oxide resins; cellulosic resins;vinyl polymers; fluoropolymers; polystyrenes; polyolefins; polysulfones;polyether sulfones; polyether ketones; polyether imides; polyphenylenesulfides; polyarylene ester resins; polyesters; homopolymers orcopolymers of N—H and/or N-alkyl glutarimide;acrylonitrile-butadiene-styrene resins; styrene-acrylonitrile resins;styrene-maleic anhydride resins; imidized styrene-maleic anhydrideresins; polyamides; polycarbonates; polycarbonate-polyesters;polyarylates; liquid crystal polymers; and mixtures thereof.
 3. The3-dimensional optical storage medium of claim 2, wherein thethermoplastic resin is selected from the group consisting of:polycarbonates; linear acrylic homopolymers and copolymers; cyclicpolyolefins; and linear imidized acrylic homopolymers and copolymers. 4.The 3-dimensional optical storage medium of claim 3 wherein the resin isa polycarbonate or polycarbonate-polyester comprising of one or morebisphenols selected from the group consisting of bis(4-hydroxyphenyl)alkanes and cycloalkanes; bis(3-substituted-4-hydroxyphenyl)alkyl-cycloalkanes; bis(3,5-disubstituted-4-hydroxyphenyl)alkylcycloalkanes; 2,2-bis(3-methyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane;1,1-bis(4-hydroxyphenyl)cyclohexane;2,2-bis(3-phenyl-4-hydroxyphenyl)propane;2,2-bis(3-isopropyl-4-hydroxyphenyl)propane;2,2-bis(4-hydroxyphenyl)butane; 9,9-bis(4-hydroxyphenyl)fluorene;9,9-bis(4-hydroxy-3-methylphenyl)fluorene; 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,1-bis(4-hydroxyphenyl)-3,3-dimethylcyclohexane;1,1-bis(3,5-dimethyl-4-hydroxyphenyl)-3,3,5-trimethylcyclohexane;1,3-bis(4-hydroxyphenyl)-5,7-dimethyladamantane;1,1-bis(4-hydroxyphenyl)-1-phenylethane;4,4′-dihydroxy-tetraphenylmethane; 2,2-bis(4-hydroxyphenyl) propane;6,6′-dihydroxy-3,3,3′,3′-tetramethyl-1,1′-spiro(bis)indane; and mixturesthereof.
 5. The 3-dimensional optical storage medium of claim 3, whereinthe thermoplastic resin is bisphenol A polycarbonate.
 6. The3-dimensional optical storage medium of claim 1 further comprising aprotective layer.
 7. The 3-dimensional optical storage medium of claim 6wherein the protective layer is laquer or resin.
 8. The 3-dimensionaloptical storage medium of claim 1 further comprising information encodedthereof.
 9. The 3-dimensional optical storage medium of claim 1 whereinthe sheet has a waviness of less than or equal to 2.0 m in a profileover a 20 mm sampling length.
 10. The 3-dimensional optical storagemedium of claim 1 wherein the sheet has a thickness variation of 10% orless over a sample length of 400 mm.
 11. The 3-dimensional opticalstorage medium of claim 1 wherein the sheet has a shrinkage of less thanor equal to 0.10%.
 12. A 3-dimensional optical storage medium comprisinga high quality plastic sheet having dispersed thereon one or morepigments, dyes or mixtures thereof, wherein the pigments, dyes ormixtures thereof undergo a change in optical properties locally uponexposure to light; wherein the optical storage medium is produced by thesteps of: a) providing molten plastic resin; b) directing the moltenplastic resin to an overflow die having an inlet and an outlet; c)shaping the molten plastic resin into a molten web using said overflowdie; d) combining the pigment, dye or mixture thereof with the moltenplastic resin prior to shaping the molten resin into a web using saidoverflow die; e) guiding said molten web away from said overflow die;and f) cooling said molten web to form a solid; wherein the high qualityplastic sheet has a shrinkage of less than or equal to 0.10%.
 13. A3-dimensional optical storage medium comprising a high quality plasticsheet having dispersed thereon one or more pigments, dyes or mixturesthereof, wherein the pigments, dyes or mixtures thereof undergo a changein optical properties locally upon exposure to light; wherein theoptical storage medium is produced by the steps of: a) providing moltenplastic resin; b) directing the molten plastic resin to an overflow diehaving an inlet and an outlet; c) shaping the molten plastic resin intoa molten web using said overflow die; d) combining the pigment, dye ormixture thereof with the molten plastic resin prior to shaping themolten resin into a web using said overflow die; e) guiding said moltenweb away from said overflow die; and f) cooling said molten web to forma solid; wherein the high quality plastic sheet has a thicknessvariation of 10% or less over a sample length of 400 mm.